In a 10-week study, 10 of 19 runners who transitioned to doing some of their mileage in Vibram FiveFingers showed signs of foot bone injury at the end of the study, compared to one of 17 runners who continued to do all of their running in conventional shoes.

For the study, to be published in Medicine & Science in Sports & Exercise, researchers in Utah divided recreational runners (average age of 26, weekly mileage of 15-30) into two groups. One group was told to continue their normal training in their conventional running shoes. The other group also continued their normal training, except that they started doing some of their running in Vibram FiveFingers. None of the study participants had previously run in FiveFingers, and none had been prevented by a lower-leg injury from running at least three days a week in the six months before the study.

The FiveFingers group followed a conservative transition plan based on one offered at the time of the study (early 2011) on Vibram's website. In the first week, they did one short run (1-2 miles) in Vibrams. During the next two weeks, they added another short run in Vibrams each week; that is, by the third week they were to do three runs of at least 1 mile in Vibrams. After the third week, they were told to increase the amount they ran in Vibrams as felt comfortable, with the goal of replacing one short run a week in conventional shoes for a short run in Vibrams.

At the start of the study, the researchers took MRIs of the runners' feet. The MRIs were repeated after the study. There were no differences post-study between the groups in what the MRIs showed about the runners' soft tissues (Achilles tendon, plantar fascia, etc.).

The bone readings were another matter.

The MRIs looked at bone marrow edema, or excess fluid in bone marrow, in several spots of the runners' feet. Assessing bone marrow edema is a common way of looking for bone injury, as the swelling in the bone marrow can indicate a protective response to injury. The researchers used a scale of 0 to 4 to rate bone marrow edema, with 0 indicating normal bone health and 4 indicating a stress fracture. Edema in 25% to 50% of bone was considered a stress reaction, and corresponded to a score of 2. The researchers considered a reading of 2 or higher to mean that the runner was injured.

By that measure, the researchers considered 10 of the 19 runners in the Vibrams group to have a foot bone injury at the end of the study, compared to one of the 17 runners in the conventional-shoes group. Two of the 10 injured Vibrams runners developed full-blown stress fractures (one in the heel, one in the second metatarsal).

This study supports the idea that, while running in barely there shoes can strengthen lower leg and feet muscles, the lack of cushioning can increase risk of bone injury.

It's worth noting that in their peak mileage week during the 10-week study, the conventional-shoe runners reported more average mileage (30) than either the Vibrams runners who didn't get injured (about 18 miles) or the Vibrams runners who got injured (about 15 miles).

The researchers concluded that, to reduce risk of foot bone injury, runners transitioning to Vibram FiveFingers should take even longer than the 10-week plan used in this study. They also advised doing all introductory runs at low intensity, which reduces impact forces. It's logical to extend their advice to other barely there minimalist shoes.